1. Field of the invention
The present invention relates to a method and an apparatus for use in a borehole, and more particularly, for carrying out nuclear measurements in a tubing of an oil producing borehole.
2. Description of the related art
As known in the art of logging techniques, a well which has been determined to be promising for oil production, is provided with a metallic casing, and cement is injected between the earth formation and the casing. Perforations are then made through the casing/cement and in the oil productive formation, so as to allow oil to flow up to the surface through a tubing beforehand arranged in the well coaxially to the casing.
It is common that oil comes up to the surface mixed with water from the formations or with water injected in the formation. As a matter of fact, water is sometimes used for enhancing the oil production in so-called "water-drive" reservoirs, wherein water is injected in the formations producing oil through wells located in the vicinity of the oil well, in order to increase the pressure in the productive formations. Water from the formations, or water injected in water-drive reservoirs, usually contains different dissolved salts which regularly become deposited on the inside wall of the production tubing as well as of the surface production equipment. These deposits, called "scales" in the oilfield production business, include sulfates such as barium, strontium or calcium, or carbonates such as calcium or iron. The scales form a layer the thickness of which increases with time, up to the point where the scales may eventually clog the production pipes and equipment.
Thus, there is a growing interest in the oilfiled production business for detecting and quantifying these scales.
Furthermore, these scales generally include radioactive components such as radium-226 from the uranium decay series or radium-228 from the thorium decay series. The protection of people who have to work in the vicinity of such contaminated equipment raises a first concern. A second concern comes from the fact that tubings are pulled out of old wells which are no longer productive, and are submitted to specific treatments so as to be re-usable. Due to the presence of radioactive scales, the removal from the well, the transportation and the treatment of the contaminated tubings is accordingly substantially complicated. Specific precautions have to be taken during each of these steps. More particularly, the contaminated tubings can only be treated in a limited number of authorized locations. A further issue is the determination of whether a tubing is contaminated or not, and, in the affirmative, to what extent. These issues are of great concern as mentioned in the two following papers: "Radioactive-Scale Formation" from A. L. Smith, published in Journal of Petroleum Technology, June 1987, pages 697-706, and in "Natural Radioactive Scale: The Development of Safe Systems of Work" from I. M. Waldram, published in the Journal of Petroleum Technology, August 1988, pages 1057-1060. These papers provide detailed information about the formation of radioactive scales, as well as information on the methods and means, available so far, to detect and measure such scales. Up to the present time, according to applicant's knowledge, there is no available straightforward and accurate method allowing such determination. The known methods and apparatus generally consist in an adaptation of existing gamma radiation meters used in other application areas, such as laboratories and hospitals.
These known apparatus are generally not sensitive enough to detect the relatively low-level radiation emitted by scales. Attempts have been made to overcome this difficulty by increasing the size of the detecting apparatus. However, this leads to bulky devices.
Furthermore, the known apparatus are relatively fragile and may be easily damaged in the severe environment of an oil production site, especially offshore.
Moreover, the known apparatus require special skills from the operator for the operating step as well as for the interpretation of the results.
Finally, since the radioactivity is measured from the outside of the tubing, these measurements can only be carried out once the tubing has been pulled out of the well. Nevertheless, there are instances where the tubing is so contaminated that it cannot, from an economical standpoint or for other reason, be decontaminated and then has to be either put back down in the well or transported to some specific locations for burial. Since these operations are very expensive, it would often be cheaper and easier to leave the tubing in the well, as long as there is no bar from the environmental regulations in force at the well location. Accordingly, it would be more appropriate to carry out the radioactive measurements inside the tubing while it is still in the well, instead of outside the tubing once it is pulled out of the well. However, the concerned people are interested in knowing the radioactivity level present outside the tubing to determine whether the tubing is a hazard if removed from the well. The known methods and apparatus do not provide any answer to the problem of detecting and quantifying the radioactivity level of a tubing disposed in a well and providing data on the radioactivity level outside the tubing.
According to the above, there is a need for a method and a tool for detecting and determining the radiation level of scales deposited on the wall of tubing.